Not applicable.
The present invention relates to a catalyst preparation and hydrocarbon conversion process used for upgrading natural gas into more valuable hydrocarbons. The invention specifically relates to a process for the conversion of hydrocarbon feedstock containing a major proportion of methane into liquids rich in aromatic hydrocarbons such as benzene, toluene, xylene and naphthalene. The scope of the invention preferably comprises the non-oxidative aromatization of methane using a crystalline aluminosilicate molecular sieve catalyst exhibiting a high conversion and a high selectivity to such aromatic hydrocarbons. Such crystalline aluminosilicate molecular sieve catalysts preferably contain molybdenum carbide or oxycarbide, have high stability, and are suitable for multiple regeneration.
There has been recognition in the prior art that it is desirable to convert methane into a higher molecular weight hydrocarbon. For instance, it is known that it is normally commercially unfeasible to transport methane produced with crude oil or natural gas from a well site to a distant location for consumption as fuel. Often the transportation problems relate to the extremely low temperatures needed to liquefy methane or to liquefy a gas mixture containing large amounts of methane. It is normally very costly to separate other light hydrocarbons such as ethane from methane and it is undesirable to admix methane with other hydrocarbons prior to transport. The result has been that large amounts of methane are essentially disposed of in a wasteful manner as by flaring without utilization of the hydrocarbonaceous nature of the methane.
U.S. Pat. No. 4,567,311 issued to L. DeVires et al. teaches the recognition of the general problem of methane utilization and also for its presentation of a process for the upgrading of methane using a specific silicon-containing catalyst. This reference provides an excellent discussion of the problems involved with methane utilization and provides a summary of the prior art relating to other methods of converting methane to ethylene or other C2+ hydrocarbons.
U.S. Pat. No. 4,565,897 issued to B. R. Gane et al. teaches the conversion of C2+ hydrocarbons using a catalyst comprising a ZSM-5 variety zeolite and gallium.
U.S. Pat. No. 4,654,455 issued to T. Chao teaches the preparation and use of a catalyst that comprises a phosphorous-containing alumina, a gallium component and a crystalline aluminosilicate such as a ZSM zeolite. The reference is directed to the conversion of C2-C5 aliphatic hydrocarbons to aromatic hydrocarbons.
U.S. Pat. No. 4,727,206 issued to D. M. Clayson et al. teaches the conversion of methane to aromatic hydrocarbons at 600-800 degrees C. using a catalyst that comprises an aluminosilicate such as a ZSM-5 variety zeolite, which has been exchanged or loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table. The preferred group VIIB metal is rhenium. This reference also teaches, at line 39-41 of column 1, that the feedstream may contain at least 50 percent and preferably at least 70 weight percent methane. The three examples all specify the aromatization of methane by contacting the catalyst with methane. Dantsin and Suslick, in an article entitled xe2x80x9cSonochemical Preparation of a Nanostructured Bifunctional Catalystxe2x80x9d, in the Journal of the American Chemical Society, Communications to the Editor, teach preparation of xe2x80x9ceggshell catalystsxe2x80x9d for the dehydroaromatization of methane to benzene, where the outer surface of the support holds nanometer sized catalyst particles. The catalyst of this reference has essentially all of the active metal on the outer surface of a ZSM-5 support relative to the pores.
It has been found that methane can be aromatized using a catalyst composition comprising a molybdenum-loaded crystalline aluminosilicate molecular sieve exhibiting the MFI crystal structure. The process of the invention preferably operates at reaction temperatures of 600-800xc2x0 C. in the absence of oxygen.
Accordingly, the present invention comprises preparation of a molybdenum-loaded crystalline aluminosilicate molecular sieve that exhibits the MFI crystal structure and has a silica-to-alumina ratio of about 50:1, said crystalline aluminosilicate preferably having the external surface acidity selectively passivated by means of an amorphous silica layer.
In another aspect, the invention comprises a one or multi-step process that contacts a feed stream, comprising at least methane, with a catalyst composition comprising a molybdenum-loaded crystalline aluminosilicate molecular sieve that exhibits the MFI crystal structure and has a silica-to-alumina ratio of 50:1, said crystalline aluminosilicate preferably having the external surface acidity selectively passivated by means of an amorphous silica layer, at hydrocarbon conversion conditions that include a temperature of 600-800xc2x0 C., a pressure of less than 5 atmospheres absolute and a Weight Hourly Space Velocity (WHSV) of 0.1-10 hxe2x88x921, and recovering product C6-plus hydrocarbons by means of an intermediate separation step.